Scientific Method —

Identifying the birthplace of near-earth asteroids

An international team of scientists looked at the structure of potentially …

Of all of the doomsday scenarios out there, asteroid impact is one of the most likely. In fact, asteroid impacts appear to have contributed to a number of previous mass extinctions, including the one that wiped out the dinosaurs. If you've seen the recent documentary on the subject, Armageddon, you already know the dangers we face.

The difference between now and then is that humanity currently has the means to identify and predict hazardous objects and, in the future, may be able to take measures to prevent catastrophic impacts. However, in order to divert or destroy an incoming body, we must understand their chemical composition and structure. In this week's Nature, an international team of scientists describe the likely structure of potentially hazardous asteroids.

Asteroids are the celestial bodies most likely to cause a catastrophic impact because many fall into the size range necessary to both generate severe damage and be attracted by Earth's gravity. In fact, many asteroids lie near earth; those that spend part of their orbit within 1+0.2 astronomical units (AU) of the sun are classified as near-Earth asteroids (NEAs). The subset of those asteroids which come within 0.05 AU of earth itself are classified as potentially hazardous asteroids (PHAs). It is generally accepted the those PHAs greater than 1 km in diameter would be seriously hazardous to life on the planet.

Researchers from the European Space Agency, Massachusetts Institute of Technology, the Paris Observatory, Johns Hopkins Applied Physics Lab, and the University of Hawaii used the SpeX instrument at NASA's Infrared Telescope Facility to study the near infrared spectra of 38 NEAs, 12 of which fall into the PHA category. The researchers then compared these to spectra obtained from a database of meteorites with known chemical composition and structure. They found that the composition of many NEAs is nearly identical to the LL chondrite class of meteorites, which have a hard, rock-like structure composed predominantly of olivine and orthopyroxene.

LL chondrites make up only 8 percent of the meteorites on earth, so the majority of hazardous asteroids originate somewhere different in the solar system than the smaller objects that have hit earth. An orbital resonance (ν6) that is extremely effective for accelerating objects toward the center of the solar system lies at the edge of the inner asteroid belt, and it is extremely likely that many NEAs originate here. Since the Flora family is located next to this resonance and matches the composition of most NEAs, it is highly likely that it is the source of many NEAs and PHAs. Armed with this new information, we can both formulate more precise strategies for destroying or diverting incoming asteroids, and keep a closer eye on their likely birthplace.